1. Field of the Invention
The invention relates to a method for manufacturing an electrode device, in particular a multipole, for use in a mass spectrometer, whereby the electrode device comprises at least a main filter and at least a pre- and/or postfilter. The invention further relates to such an electrode device, as well as a mass spectrometer with such a multipolar electrode device.
2. Description of the Related Art
Multipolar electrode assemblies for the characterization of the chemical compounds are known in the prior art, for example from German patent specification 944 900. Such multipole mass filters work without a magnetic field. A quadrupole, for example, consists of four metal rods, which serve as electrodes and which are arranged on a circle with radius R0. The voltage at the electrodes is composed of a high-frequency AC voltage and a DC voltage, where the respective opposite pairs of the electrodes have a high-frequency voltage shifted by 180°. The ions to be separated are shot in longitudinal direction of the electrodes into the field as a fine ion beam. Due to the applied AC and DC voltages, the ions are moved along defined trajectories through the mass filter. Outside stable basic conditions the ions collide with the electrodes, whereby the ions are neutralized. This results in that these neutralized ions no longer get to the detector.
The voltage applied to the electrodes is linear to the detected ion mass, wherefore for running through the mass range, thus for adjusting the desired mass to be detected, a proportional change of AC and DC voltage has to be made. A change of the resolution can be caused by varying the voltage conditions. In particular, a stability diagram is important, which is calculated according to the differential equations of Mathieu. A good overview about the functioning of a quadrupole including an explanation of the stability diagram can be found in Miller & Denton, 1986, “The Quadrupole Mass Filter: Basic Operating Concepts”, Journal of Chemical Education, Volume 63, No. 7, pages 617 to 623.
When measuring with a multipole, particularly the alignment of the electrodes to each other is important because the alignment must be executed with high accuracy. A manufacturing process for this high-precision alignment is known, for example, from DE 10 2004 054 835 A1. Nevertheless, the problem remains that the border areas of the electrodes represent more unstable zones for ions and thus contribute to a defocusing. This effect was particularly studied by Dawson, 1971, “Fringing Fields in the Quadrupole Mass Filter”, International Journal of Mass Spectrometry and Ion Physics, Volume 6, pages 33 to 44. Dawson did simulations based on the findings of Brubaker, who proposed prefilter and postfilter for the first time. Prefilters and postfilters act as a pre- or poststage of the main filter by only applying an extenuated AC voltage. Thus, the field begins and ends not abruptly for the ions, but the ions are slowly led out or led into the field. Therefore, the ions reach a higher stability and thus a better focusing. Thereby, the prefilters and postfilters work like a lens.
The high-precision alignment of the electrodes to each other (for example, prefilter to main filter) has to be considered in the implementation of prefilters and postfilters in practical use because even small inaccuracies can lead to field interference. In DE 22 15 763, for example, the implementation of prefilters and postfilters is shown. However, in this case great effort with high costs are involved for high-precision alignment of the filters to each other. Therefore, in the prior art a method is missing, which ensures an accurate alignment with low effort and thus also has a high analytical measurement accuracy.